Development of a nitride semiconductor device as a high-voltage, high-power semiconductor device by taking advantage of features such as a high saturated electron velocity and a wide band gap is active and ongoing. Nitride semiconductor devices include field-effect transistors. There have been numerous reports on field-effect transistors, particularly high electron mobility transistors (HEMTs). Among HEMTs, an AlGaN/GaN HEMT using GaN in an electron transit layer and AlGaN in an electron supply layer is attracting attention. In an AlGaN/GaN HEMT, distortion occurs in AlGaN due to the difference in lattice constants between GaN and AlGaN. Piezoelectric polarization resulting from the distortion and spontaneous polarization of AlGaN lead to formation of a high concentration of two-dimensional electron gas (2DEG). Accordingly, high voltage and high power can be achieved. [0004] Patent Literature 1: Japanese National Publication of International Patent Application No. 2009-524242
Operating voltage needs to be increased in a nitride semiconductor device for high output and high frequency applications, such as an AlGaN/GaN HEMT, to obtain high power. An increase in operating voltage, however, increases the electric field strength at and around a gate electrode and causes deterioration in device characteristics (chemical and physical changes). In order to enhance the reliability of a high-power nitride semiconductor device, it is essential to inhibit deterioration in device characteristics caused by a high electric field at and around a gate electrode.